This is a proposal for an Exploratory Research Grant (R21), submitted in response to RFA DK 98 009 on the Pathogenesis and Therapy of Complications of Diabetes. The long-term goal of the proposed research is to understand the role of histamine released from centrifugal axons in the etiology of small vessel damage to the retina of diabetic patients. In preliminary experiments, axons from the optic nerve, but not cell bodies, in the macaque retina were found to contain histamine, and their morphology and contacts with retinal blood vessels were described more completely than was possible with other, less-selective staining methods. These results provide additional evidence that centrifugal axons actually exist in primates. The only neurons in the brain that contain histamine are found in the posterior hypothalamus, where retrograde labeling studies indicate that the centrifugal axons originate. The results also suggest that histamine and its antagonists could be used to study the functions of the centrifugal axons in the normal primate retina, and they suggest explanations for the histamine effects that have been observed already. Finally, these findings may be clinically-significant since histamine antagonists apparently prevent the breakdown in the blood-retina barrier in diabetic patients. The first specific aim of the proposed experiments is to develop a method for labeling the centrifugal axons for electron microscopy and to study their ultrastructure. The exact number and diameters of the centrifugal axons in macaque retinas will be determined, and if they make specialized contacts with retinal blood vessels or retinal neurons, these will be described. Three possible marker for electron microscopic immunohistochemistry are: the neuropeptide galanin, which has been localized to histamine-containing neurons in primates and to axons, but not cell bodies, in the macaque retina; histidine decarboxylase, the enzyme that synthesizes histamine, and histamine, itself. All three have been localized previously by electron microscopy, but not in the retina. The second specific aim is to determine whether human centrifugal axons also contain histamine, as expected from the preliminary data, and to describe any changes in these axons in eyes of diabetic donors. The working hypothesis is that these axons are more active than normal in diabetics and that this is why antihistamines are effective in preventing vascular leakage in these patients. If this is correct, there may be a morphological correlate of this increased activity. The human material will also be studied using the electron microscopic technique developed in the first set of experiments.